Broad Institute MLPCN HIV Entry Inhibitor Probe Project

None of the currently available HIV-1 entry inhibitors represents a strong candidate for an effective prophylactic microbicide. The work proposed in this project seeks to identify lead HIV-1 entry inhibitors that are potentially suitable for development as preventive prophylactic agents. We would also welcome the discovery of compounds that are useful in treatment settings or as probes to more ..

None of the currently available HIV-1 entry inhibitors represents a strong candidate for an effective prophylactic microbicide. The work proposed in this project seeks to identify lead HIV-1 entry inhibitors that are potentially suitable for development as preventive prophylactic agents. We would also welcome the discovery of compounds that are useful in treatment settings or as probes to dissect the HIV-1 entry mechanism. The probe generated should meet the following criteria: the probe should have an IC50 of less than 1 uM in the primary assay. The probe should possess activity against all 4 major clades of virus (A,B,C,D) inhibiting 90% of virus infection in at least one member of each clade. HIV-1 isolates resistant to the probe should have more than 2 genetic mutations to achieve drug resistance from the probe. If the probe is an inhibitor or binder of gp120 or inhibitor of envelope conformational changes, the probe must have greater cell-based efficacy and/or breadth than BMS-488043 and PF-68742, respectively. The probe should not be cytotoxic to any of the cell lines used in the assay and exhibit an IC50>50 fold the primary assay IC50. Finally, the compound should be soluble in the cell culture media or assay buffers used to evaluate the compounds.

HIV, Env, cell fusion, viral entryBiological Relevance:Human immunodeficiency virus (HIV-1) is the etiologic agent of acquired immunodeficiency syndrome (AIDS). The global HIV-1 pandemic, in which ~35 million people are infected, is sustained by 2-3 million new infections each year. Changing the course of the AIDS pandemic requires prevention of HIV-1 transmission, most of which occurs sexually. Sexual transmission of HIV-1 is an inefficient process, with most recipients becoming infected by a single virus. In the absence of a vaccine, disrupting sexual transmission with specific antiretroviral agents represents an attractive approach to HIV-1 prevention. However, partial efficacy due to the existence of drug-resistant viruses and the risk of systemic side effects represent barriers to the general applicability of this approach to HIV-1 prophylaxis. New agents that are safe, effective against the great variety of existing HIV-1 strains, and readily formulated as microbicides are needed for prevention of HIV-1 sexual transmission.

The HIV-1 envelope glycoproteins (Envs), which mediate virus entry into target cells, represent attractive targets for such prophylactic agents. The HIV-1 Envs are assembled into trimeric spikes on the viral surface, with each spike composed of three gp120 exterior Envs and three gp41 transmembrane Envs. The gp120 Envs bind the receptors, CD4 and CCR5/CXCR4, on the target cell. The HIV-1 Envs on the viral surface, as well as the host cell receptors, are potentially accessible to water-soluble inhibitors. Thus, for Env-targeted inhibitors, even compounds that are not taken up by host cells or absorbed systemically can be used as microbicides. This limits the potential toxicity of these compounds and expands the range of molecules that can be used.The HIV-1 Env trimer is synthesized as a high-potential-energy complex that is triggered by receptor binding to mediate the fusion of the viral and cell membranes. Receptor binding induces conformational changes in the Env trimer, culminating in the formation of a six-helix bundle in gp41 that drives the fusion of the viral and target cell membranes. The high potential energy and surface exposure of the HIV-1 Envs create opportunities for inhibition by two means: 1) premature activation, which in the absence of a target membrane leads to irreversible inactivation; and 2) classical inhibition, where a small molecule interferes with a process required for virus entry (e.g., receptor binding, receptor-induced conformational changes, six- helix bundle formation). A hurdle for all HIV-1 inhibitors is the ability of the virus to generate variants that, in some cases, exhibit decreased sensitivity to the antiviral agent. Under the selective pressure of potentially neutralizing antibodies, the Env spike of different HIV-1 strains exhibits significant variation in sequence and pattern of glycosylation. This variation, however, is mainly confined to specific surface-exposed loops potentially accessible to antibodies. Elements of the HIV-1 Envs involved in receptor binding, conformational rearrangements, and membrane fusion exhibit a high degree of conservation and, although hidden from antibodies, are potentially accessible to small-molecule inhibitors.